1. Field of the Invention
The present invention relates to an antilock brake control system for a vehicle, including wheel speed detecting means for detecting a wheel speed, vehicle speed calculating means for calculating a presumed vehicle speed based on the wheel speed detected by the wheel speed detecting means, slip rate calculating means for calculating a slip rate based on the wheel speed detected by the wheel speed detecting means and the presumed vehicle speed calculated in the vehicle speed calculating means, and a control start determining means for calculating a start determination threshold value as a function of the presumed vehicle speed calculated in the vehicle speed calculating means and for determining that an antilock brake control should be started, in response to the slip rate calculated in the slip rate calculating means exceeding the start determination threshold value.
2. Description of the Related Art
An antilock brake control system is already known, for example, from Japanese Patent Application Laid-open No. 3-16866, in which the condition for starting the antilock brake control is when the slip rate exceeds the start determination threshold value.
In the above known system, however, the start determination threshold value is set constant irrespective of the situation of a road surface, and an antilock brake control suitable for a variation in situation of the road surface cannot be performed. Thus, it is desirable that when the friction coefficient of a road surface is relatively low, the start determination threshold value for determining the start of the antilock brake control is set at a relatively low value in order to enhance the stability of the vehicle body. However, if the start determination threshold value is set at a relatively low value during traveling of the vehicle on a bad road, the turning ON and OFF of the antilock brake control may be frequently repeated in some cases due to undulation on a road surface, thereby providing discomfort to an occupant.